The present invention relates to methods and apparatus for fracturing rock, ceramics, concrete and other materials of low elasticity. The invention relates in particular to methods and apparatus for fracturing rock for purposes of mining, excavation, and demolition.
Mining and excavation of rock is commonly carried out using explosives. Typically, sticks of explosive are placed in holes drilled into the rock and then detonated, thereby explosively fragmenting a portion of the rockface being worked on. The rock debris created by the explosion is cleared away, and preparations begin for another blast.
The blasting method described above is time-consuming and expensive. Each blast takes a considerable time to set up and carry out. A large number of holes must be drilled into the rockface, then the explosives placed in the holes must be carefully interconnected with fusing apparatus to ensure that they detonate simultaneously. The resultant blast can throw rock debris large distances, unless the configuration of the blast is such that heavy and expensive blasting mats can be put in place to cushion the explosion and prevent the blast debris from flying away. As with any operation employing explosives, the blasting method also is inherently hazardous to the persons involved.
Accordingly, there is a need for rock mining and excavation methods which are faster and more efficient and thus less expensive than conventional blasting methods. There is also a need for rock mining and excavation methods which eliminate or substantially reduce the safety hazards associated with conventional rock blasting practices.
One possible alternative to conventional mining methods is to fracture the rock by means of thermal stress. It is well known that solid materials can fracture due to internal stresses induced by a large and sudden temperature change. A simple example of this is the shattering of a piece of glassware plunged into cold water after having been heated. Similarly, rock will shatter if it undergoes a temperature rise great enough and sudden enough to induce internal tensile stresses exceeding the inherent tensile strength of the rock. This would be a desirable result for purposes of rock mining and excavation. Material near the surface of a rock mass would be heated rapidly, and resultant thermal stresses would fracture the rock. The fractured material would be removed, then the process would be repeated on the fresh rock thus exposed, and so on until a desired amount of rock has been removed.
The practical difficulty with this concept, of course, is how to create such a sufficiently sharp and intense temperature rise in the surficial zone of a rock mass, before the heat thus transferred to the rock can be dissipated by conduction throughout the rest of the rock mass. One obvious aspect of the solution is to use an extremely hot source of heat. Conventional flame-heat sources, however, are not capable of achieving the desired result. An acetylene-oxygen flame, for example, can achieve a maximum temperature of approximately 3,100 degrees Celsius, but tests have indicated that even a flame this hot is not effective for producing thermal stresses intense enough to fracture rock.
U.S. Pat. No. 4,027,185 issued to Nodwell et al. on May 31, 1977, U.S. Pat. No. 4,700,102 issued to Camm et al. on Oct. 13, 1987, and U.S. Pat. No. 4,937,490 issued to Camm et al. on Jun. 26, 1990, the contents of which are incorporated herein by reference, disclose closely similar arc lamps capable of generating white light at temperatures as high as 12,000 degrees Celsius, considerably hotter than the temperatures which can be achieved with flame heat. These arc lamps have been developed and used for such applications as simulating, for purposes of scientific experiments, the high temperatures produced by nuclear explosions. The white light generated by these arc lamps is hot enough to heat rock high enough and quickly enough to produce thermal-stress-induced fracture, and in fact is capable of heating an object a great deal faster than a flame source.
This can be illustrated by the well-established heat transfer equation for radiant heat, as follows:
Q="sgr"E F A (T14xe2x88x92T24) 
wherein:
Q=amount of heat transferred
"sgr"=Stefan-Boltzmann constant
E=emissivity
F=shape factor
A=area
T1=temperature of heat source
T2=initial temperature of heat absorber (i.e., object being heated)
This equation may be used to compare the amounts of heat transferred to an object by a white light source and by a flame source. Factors "sgr", E, F, and A will be constant for each case. Given that T1 will be far greater than T2 in either case, it is evident on inspection that the term (T14xe2x88x92T24) may be reduced to merely T14 without significant loss of accuracy. It follows, therefore, that:
QL/QF=TIL4/TIF4=(TIL/TIF)4 
where:
QL=amount of heat transferred to heat absorber by light source
QF=amount of heat transferred to heat absorber by flame source
TIL=temperature of light source
TIF=temperature of flame source
Therefore, if the temperature of the light source is 12,000 degrees Celsius, and the temperature of the flame source is 3,100 degrees Celsius, the heat transfer from the light source will be (12,000/3,100)4 or 225 times that of the flame source.
White light arc lamps of the type taught by Nodwell et al. and Camm et al. feature a hollow, elongate quartz arc chamber positioned within an elongate concave reflector. The reflector is hollow, so that liquid coolant may be circulated through the reflector to prevent it from becoming overheated under the intense heat generated by the arc chamber. For proper operation, this type of arc lamp requires an extremely clean environment. Even tiny amounts of dust or dirt on the quartz arc chamber or the reflector can cause the lamp to fail, or to function with significantly reduced effectiveness.
For these reasons, white light arc lamps have typically been used only in controlled environments such as experimental laboratories. If used, unmodified, for thermal-stress-induced fracturing of rock, they would likely malfunction because of the dirty air typically associated with rock mining and excavation operations. One apparent possible solution to this problem would be to enclose the arc chamber and reflector inside a translucent cover, thereby shielding them from airborne particles while allowing light to pass through. The solution cannot be quite this simple, however; airborne particles would build up on the cover, melt under the intense heat from the lamp, and interfere with the transmission of light from the lamp. Therefore, any cover over the arc chamber and reflector would have to be kept extremely clean, even in a dirty environment.
Accordingly, there is a need for an improved white light arc lamp, the arc chamber and reflector of which will remain clean and effectively dust-free even in environments having significant concentrations of airborne particulate matter. As well, there is a need for an improved white light arc lamp having means for keeping the arc chamber and reflector clean in dirty environments while also ensuring effectively unimpeded transmission of light from the arc lamp to a target object.
In one aspect, the present invention is the use of high-intensity white light to induce thermal stress fracture in brittle materials such as rock, ceramics or concrete. In another aspect, the present invention is a method of fracturing brittle materials such as rock, ceramics or concrete, comprising the step of directing white light generated by a high-intensity arc lamp upon a mass of the brittle material until the material fractures due to induced thermal stresses.
In another aspect of the invention, the invention comprises a high intensity arc lamp for generating and directing high intensity light toward a target object, said arc lamp having an arc chamber and comprising:
(a) a convex reflector enclosure which partially encloses the arc chamber and which comprises an air inlet;
(b) an air plenum associated with the enclosure;
(b) a source of air for introduction into the air plenum;
(c) filtering means for filtering particulate matter from the air before it is introduced into the air plenum;
(d) a fan for forcing air from the air plenum through the air inlet segmented reflector and past the arc chamber, so as to create an air shield travelling outwardly away from the arc chamber and the reflector and deflecting airborne particulate matter away from the arc chamber and the reflector; and
(e) cooling means for cooling the reflector and peripheral surfaces of the air plenum.
Preferably, the convex reflector is divided into at least two segments which are spaced apart and the air inlet is the space(s) between the segments of the reflector. More preferably, the convex reflector comprises at least three longitudinal segments thereby providing at least two longitudinal air inlets between the longitudinal segments.
In another aspect, the invention comprises an apparatus for shielding the arc chamber and reflector against the entry and build-up of airborne particulate matter, for use in a high-intensity arc lamp having an elongate arc chamber positioned within an elongate concave reflector, said apparatus comprising:
(a) a translucent cylindrical shield mounted to the arc lamp so as to encircle and enclose the arc chamber and reflector, with the longitudinal axes of the translucent cylindrical shield and the arc chamber being substantially coincident or parallel;
(b) means for rotating the translucent cylindrical shield about its longitudinal axis; and
(c) means for continuously cleaning the surfaces of the translucent cylindrical shield as it rotates.
In yet another aspect, the invention comprises an apparatus for shielding the arc chamber and reflector against the entry and build-up of airborne particulate matter in a high-intensity arc lamp having an elongate arc chamber positioned within an elongate concave reflector, said apparatus comprising:
(a) a first shield chamber associated with one longitudinal edge of the reflector;
(b) a second shield chamber associated with the other longitudinal edge of the concave reflector;
(c) a translucent planar shield approximately as long and slightly more than twice as wide as the open side of the reflector, and positioned such that it completely closes off the open side of the reflector, thereby enclosing the arc chamber, and such that the portion of the translucent planar shield not thus positioned across the open side of the reflector at a given time will be housed within either the first or second shield chamber;
(d) means for moving the translucent planar shield in a reciprocating fashion in its own plane, such that it alternately extends partially into the first shield chamber and then partially into the second shield chamber while at all times being positioned across and closing off the open side of the reflector and enclosing the arc chamber;
(e) means within the first shield chamber and second shield chamber for cleaning the surfaces of the translucent planar shield as it moves alternately into or out of the first and second shield chambers.